Red and blue light are the most important light spectra for driving photosynthesis to produce adequate crop yield. It is also believed that green light may contribute to adaptations to growth. However, the effects of green light, which can trigger specific and necessary responses of plant growth, have been underestimated in the past. In this study, lettuce (Lactuca sativa L.) was exposed to different continuous light (CL) conditions for 48 h by a combination of red and blue light-emitting diodes (LEDs) supplemented with or without green LEDs, in an environmental-controlled growth chamber. Green light supplementation enhanced photosynthetic capacity by increasing net photosynthetic rates (Pn ), maximal photochemical efficiency (Fv /Fm ), electron transport for carbon fixation (JPSII ) and chlorophyll content in plants under the CL treatment. Green light decreased malondialdehyde and H2 O2 accumulation by increasing the activities of superoxide dismutase (SOD; EC 1.15.1.1) and ascorbate peroxidase (APX; EC 1.11.1.11) after 24 h of CL. Supplemental green light significantly increased the expression of photosynthetic genes LHCb and PsbA from 6 to 12 h, and these gene expression were maintained at higher levels than those under other light conditions between 12 and 24 h. However, a notable down-regulation of both LHCb and PsbA was observed during 24 to 48 h. These results indicate that the effects of green light on lettuce plant growth, via enhancing activity of particular components of antioxidantive enzyme system and promoting of LHCb and PsbA expression to maintain higher photosynthetic capacity, alleviated a number of the negative effects caused by CL.

Most leafy vegetables can accumulate large amounts of nitrate, which are often associated with harmful effects on human health. Nitrate assimilation in plants is determined by various growth conditions, especially light conditions including light intensity, light duration and light spectral composition. Red and blue light are the most important since both drive photosynthesis. Increasingly, recent evidence demonstrates a role for green light in the regulation of plant growth and development by regulating the expression of some specific genes. However, the effect of green light on nitrate assimilation has been underestimated. In this study, lettuce (Lactuca sativa L. cv. Butterhead) was treated with continuous light (CL) for 48 h by combined red and blue light-emitting diodes (LEDs) supplemented with or without green LED in an environment-controlled growth chamber. The results showed that nitrate reductase (NR) and nitrite reductase (NiR) related-gene expression and nitrate assimilation enzyme activities were affected by light spectral composition and light duration of CL. Adding green light to red and blue light promoted NR and NiR expressions at 24 h, subsequently, it reduced expression of these genes during CL. Compared with red and blue LEDs, green light supplementation significantly increased NR, NiR, glutamate synthase (GOGAT) and glutamine synthetase (GS) activities. Green-light supplementation under red and blue light was more efficient in promoting nutritional values by maintaining high net photosynthetic rates (Pn) and maximal photochemical efficiency (Fv/Fm).